It has been reported that 2-hexene-1,6-dial is obtained by reacting 1,3-cyclohexadiene with ozone and dimethyl sulfide (see T. Hudlicky et al., Journal of the American Chemical Society, Vol. 110, p. 4735 (1988)). However, the process must be carried out by using an expensive raw material in an extremely low concentration while controlling the ozone concentration at -78.degree. C. and is not therefore deemed suitable for industrial production.
On the other hand, generally known processes for dimerizing an olefin having an electron-withdrawing substituent in a tail-to-tail fashion include a process for obtaining dimethyl dihydromuconate which comprises dimerizing an acrylic ester by using, as a catalyst, a ruthenium (see R. J. McKinney et al., Organometallics, Vol. 5, p. 1080 (1986)), a ruthenium or palladium compound (see U.S. Pat. No. 3,013,066), or rhodium (see M. Brookhart et al., Journal of the American Chemical Society, Vol. 133, p. 2777 (1991)); and a process for obtaining 1,4-dicyano-1-butene which comprises dimerizing acrylonitrile by using, as a catalyst, a ruthenium compound (see JP-A-44-24585, the term "JP-A" as used herein means an "unexamined published Japanese patent application") or an organophosphorus compound (see JP-A-55-98149).
It is also known that dimerization of 2-propenal (1-propen-3-al) at 185.degree. to 195.degree. C. in the absence of a catalyst yields 2,3-dihydro-2-formylpyran (see JP-B-40-9264; the term "JP-B" as used herein means an "examined published Japanese patent application"). Further, it is reported that polymerization of 2-propenal proceeds in the presence of a ruthenium compound as a catalyst (see S. Komiya et al, Bulletin of the Chemical Society of Japan, Vol. 48, p. 101 (1975)), but the paper gives no reference to dimerization of 2-propenal.